Field effect transistors (FETs) can be used in sensors configured to detect and analyze chemical substances and biological agents within fluids. Typically, these sensors rely on the voltage developed between the gate and a reference electrode. More particularly, changes in the gate bias result in a change in the channel current flowing through the device. In some designs, a fluid being sensed is in intimate contact with the gate dielectric. In other designs, the fluid being sensed is separated from the gate dielectric by a sensitive membrane or coating (e.g., a layer or coating applied to the gate dielectric). For example, depending on the application and the type of sensitive membrane or coating overlying the gate dielectric, these devices may be termed ion-sensitive field effect transistors (ISFETS), immunological field effect transistors (IMFETS) or enzyme field effect transistors (ENFETS).